1. Field of the Invention
The present invention concerns a method and an apparatus for transmission of signals.
2. Description of the Prior Art
In many applications, electrical as well as optical signals must be transmitted with a high dynamic range, for example in magnetic resonance imaging systems (MR), in particular in medical imaging, for example magnetic resonance tomography. MR signals exhibit a very high dynamic range of approximately 100 dB, but exhibit a relatively small monitoring bandwidth of approximately 500 kHz.
In the transmission link, the analog signals are often converted into digital (generally binary) signals by an analog-digital converter (A/D converter) and then digitally stored and/or processed further, or converted back again into analog signals by a digital-analog converter (D/A converter). An A/D converter circuit suitable for MR is known, for example, from DE 199 14 207 C2.
It is also known (for example from DE 101 48 441 C1) to transmit the analog signals via a wireless transmission link, for example for wireless readout of MR local coils or movable MR coils, by modulation and transmission of the modulated signals and subsequent demodulation of the received signals. The demodulated signals are then digitized by an A/D converter.
In order to be able to limit the digital resolution or the bit-width of the A/D converter, or the bandwidth of the (in particular wireless) transmission link, even for signals with high dynamic range, it is known (for example from DE 101 48 441 C1) to use a dynamic compressor before the A/D converter or the further (in particular wireless) transmission link. The dynamic compressor reduces or compresses the dynamic range of the signals before their digitization or transmission. The original signal is then regained by digital expansion from the stored, digital compressed signals or digital compressed signals, transmitted in another manner, for example using a value table or lookup table.
In order to retain a good signal-to-noise ratio S/N, even for small amplitude signals, generally such signals are still linearly amplified, but are only compressed at a specific level range, for example 60 dB given full modulation (maximum recording level), or above a specific threshold of the signal level. In the digitization it is thereby ensured that the A/D converter is also modulated to a sufficient degree even for small signals. The larger the signal amplitude, the more significantly it is compressed in general, i.e. the greater the degree of compression or compression factor of the dynamic compressor. In other words, the characteristic of the compressor, representing the output signal plotted with respect to the input signal, initially climbs linearly up to a threshold, and beyond the threshold the characteristic is convex or continuously decreases in the positive slope or first derivative. The characteristic above the threshold frequently has the curve of an exponential function with a positive exponent smaller than 1, typically between 0.2 and 0.5. The overall characteristic normally exhibits a point-symmetrical curve (thus two of the characteristic branches just described are mirrored point-for-point at the origin), or is an odd function, in order to achieve the same compression for positive and negative signals.
Such a dynamic compressor is described, for example, in the aforementioned DE 101 48 441 C1 and has a number of amplifier stages connected in series each stage having an output amplification limited to a stage limit level, and a summation stage for summation of the output signals of the amplifier stages and of the input signal to be compressed. The summation signal is then the compressed signal.